1. Field of the Invention
The present invention relates to an apparatus and method for mobile communication in which a base station for favorable communications is selected from a plurality of base stations.
2. Description of the Related Art
A system known as CDMA (Code Division Multiple Access) has been employed in a mobile communication apparatus. For example, in a CDMA mobile communication system, a base station is disposed in each of a plurality of service area (cells) so that wireless asynchronous communications are carried out between the base station and a communication apparatus (portable information apparatus) of a user.
A portable information apparatus that has been used until now is provided with a receiving circuit as shown in FIG. 1 which selects one base station among a plurality of base stations for satisfactory communications. The receiving circuit is provided with an antenna 1, a front end 2, an inverse diffusing circuit 3, a detecting/demodulating circuit 4, a base station selecting circuit 5, and a received power detection circuit 6. The antenna 1 and the front end 2 receive a radio wave in a down channel arrived from a base station. A received signal Sin output from the front end 2 is inverse-diffused in the inverse diffusing circuit 3. The inverse-diffused signal is further detected and decoded in the detecting/demodulating circuit 4, by which a demodulated signal is produced. Furthermore, the received power detection circuit 6 automatically measures electric power of a received signal Sin. The base station selecting circuit 5 determines a base station transmitting a radio wave with the largest measured power P as the station to communicate with, and, on the basis of a result of the determination, controls the inverse diffusing circuit 3 and detecting/demodulating circuit 4.
With thus provided received power detection circuit 6 and base station selecting circuit 5 for the base station selection, even when the portable information apparatus travels with the user from a point a, to a point b, then to a point c, and so on, as illustrated in FIG. 2, a handover (cell switching) of the portable information apparatus is carried out in the order of a base station A, B, and C, and so on, each of which is at the shortest distance from the apparatus in the traveling direction of the user. It is possible to maintain suitable communication even if the user is traveling.
In the above base station selection, of radio waves arriving from a plurality of base stations, one radio wave with the highest electric field strength is detected. Then, a base station transmitting the detected radio wave is determined to be selected as the nearest suitable base station in the traveling direction.
However, as shown in FIG. 3, a radio wave transmitted from a base station Al behind a portable information apparatus P1 which is traveling is reflected by a reflector B1 such as a building. As a result, the portable information apparatus P1 can receive the reflected radio wave with a high electric field strength. Therefore, in the above base station selection which is strongly dependent on electric field strength of a radio wave of a base station, the reflected radio wave is incorrectly determined as a radio wave transmitted from the nearest base station in the traveling direction. This causes frequent repetition of unnecessary handover that results in ineffective utilization of limited resource and of occurrence of undesirable disconnection that degrades communication quality.
In view of the foregoing, it is an object of the present invention to provide a mobile communication apparatus and method by which occurrences of unnecessary handovers can be reduced to improve communication quality.
The mobile communication apparatus according to the present invention selects any one of a plurality of base stations to communicate therewith, the apparatus comprising: extracting means for extracting surrounding base stations of a mobile unit within a certain defined area centered at a present location of the mobile unit; and base station selecting means for carrying out an arithmetic operation for obtaining a specified weighting coefficient with respect to each of the extracted surrounding base stations on the basis of a direction of each of the extracted surrounding base stations from the present location of the mobile unit and a traveling direction of the mobile unit, for carrying out further arithmetic operation for weighting a value of the electric field strength of a signal received from each of the surrounding base stations with the weighting coefficient of a corresponding surrounding base station to provide a weighted electric field strength of each of the extracted surrounding base station, and for selecting the one base station from the extracted surrounding base stations depending on a value of the weighted electric field strength.
The mobile communication apparatus according to the present invention selects any one of a plurality of base stations to communicate therewith, the apparatus comprising: a receiving section for receiving a radio wave transmitted from a satellite at a fixed time interval, and for outputting present location data in synchronism with the time interval; a present location storing section for storing the present location data supplied from the receiving section; an operation section into which location information indicating a destination is input as destination data by a user; a map information storing section for storing map information including location data of each of base stations settled in each of service areas; a traveling direction estimating section for estimating a traveling direction in which the user is going to travel, and for outputting the estimated direction as estimation direction data; and a candidate base station selecting section which receives the estimation direction data output from the traveling direction estimating section; retrieves base station location data about all of the base stations in an area within a specified radius centered at the present location from the map data storing section on the basis of the present location data; carries out an arithmetic operation for obtaining a direction of each of the base stations as a direction of a vector with an initial point thereof at a point represented by the present location data and a terminal point thereof at a point represented by a retrieved base station location data of each of the base stations, the direction of the vector being defined as a base station deflection angle which is an angle between the vector and the axis of the coordinate system; carries out an arithmetic operation for comparing each of the obtained base station deflection angles with the destination deflection angle, for obtaining weighting coefficients each corresponding to each of the base stations and providing a higher priority of being made as a candidate of the one base station for a base station with the base station deflection angle closer to the destination deflection angle, and for weighting a value of an electric field strength of a signal received from each of surrounding base stations with the corresponding weighting coefficient to provide a weighted electric field strength for each of base stations; and selects the one base station depending on a value of the weighted electric field strength.
A method for communication used in a mobile communication system according to the present invention selects any one of a plurality of base stations to communicate with a mobile unit, the method comprising the steps of: inputting a traveling destination of the mobile unit; detecting a present location of the mobile unit at a fixed time interval; carrying out an arithmetic operation for obtaining a weighting coefficient for each of base stations, the weighting coefficient being provided so that a base station at the shortest distance from the mobile unit in a traveling direction thereof is made as a candidate of the one base station; carrying out an arithmetic operation for weighting a value of an electric field strength of a signal received from each of surrounding base stations with the corresponding weighting coefficient to provide a weighted electric field strength for each of base stations; and selecting the one base station depending on a value of the weighted electric field strength.